1. Field of the Invention
The present invention relates generally to the field of bio-electrochemistry and more particularly to multi-electrode microbial fuel cells and dynamically reconfigurable systems comprising multiple such fuel cells.
2. Description of the Prior Art
A Microbial Fuel Cell (MFC) is a device that comprises an anode, a cathode, and a liquid medium including a population of microbes. In a microbial fuel cell the microbes perform electrochemical reactions to provide an electrical current through an external circuit disposed between the anode and the cathode. An example of a standard MFC 100 is shown in FIG. 1. The MFC 100 comprises a vessel 110 divided into an anode chamber 120 and a cathode chamber 130 by a semi-permeable membrane 140. The anode chamber 120 includes an anode 150 while the cathode chamber 130 includes a cathode 160. The anode chamber additionally includes a population of microbes 170. The anode 150 and cathode 160 are electrically connected through an external circuit 180. MFC 100 also comprises a solution 190 within which the anode 150 and cathode 160 are at least partially immersed and within which the population of microbes 170 is maintained. The semi-permeable membrane 140 is typically a proton exchange membrane (PEM), which provides a conduction path for hydrogen ions but not electrons, which then must travel over the external circuit 180 to reach the cathode 160.
In operation, a nutrient is added to the solution 190 and the microbes 170 consume the nutrient, under anaerobic conditions. The microbes 170 therefore obtain oxygen by splitting water into hydrogen ions, oxygen, and electrons. The oxygen is combined with the carbon from the nutrient to form CO2, the hydrogen ions migrate across the membrane 140 to the cathode 160, and the electrons traverse the external circuit 180 from the anode 150 to the cathode 160 where the electrons combine with the hydrogen ions.
There are two general types of MFCs 100, mediated and unmediated. In a mediated MFC 100 an intermediate molecule transfers electrons from the microbes 170 to the anode 150. An example of a mediator molecule is Methyl Blue. Many types of microbes can be used in mediated MFCs, including e. coli. 
Fewer types of microbes 170 can be used in an unmediated MFC 100. Unmediated MFCs 100 have, for at least part of their population, microbes 170 which can deposit electrons on the anode 150 directly, without the use of an added mediator. This capability is often, but not always, referred to as “exoelectrogenesis” and microbes 170 capable of exoelectrogenesis are referred to as “exoelectrogenic.” Different mechanisms of exoelectrogenesis exist, and the phenomenon is observed in many species of microbes 170, most of which are bacteria. In bacteria, exoelectrogenic capability is found in specific species of diverse genera. In the case of Shewanella it is believed that the bacteria produce their own mediator. In the case of Geobacter metallireducens, it has been shown that the bacteria produce conductive pili which facilitate the deposit of electrons on the anode 150. In FIG. 1 the microbe 170 is shown touching the anode 150 to illustrate the unmediated type.
MFC 100 is sometimes referred to as a “two-chambered MFC” because, as noted, the membrane 140 separates the vessel into two chambers 120, 130. Another type of MFC are the “membraneless MFCs” which lack a physical barrier such as membrane 140. Instead, hydrogen ions travel through the solution 190 to the cathode 160 while conditions are maintained that favor the movement of electrons over the external circuit 180 to prevent the electrons from reacting with the hydrogen ions in the solution 190. MFCs of this type are generally referred to as “single chamber MFCs.”
Closely related to MFCs are Microbial Electrolysis Cells (MECs) which make use of the electrons and/or hydrogen ions on the cathode-side of an MFC-like device. The nomenclature for MECs-like devices is not as well defined, so these are sometimes referred to as Biological Electrolysis Cells (BEC), Biological Electrically Assisted Microbial Reactors (BEAMR) and other names, but regardless of the name, these employ the structure of the MFC 100 except that a reaction of interest, other than the formation of H2O, occurs at the cathode, with or without microbial involvement at the anode. For the purposes of this application, the term MFC is intended to cover MECs and all pseudonyms for MECs.
A flow-through MFC (FTMFC) is a MFC where a liquid or gas enters the MFC through an inlet, exits through an outlet, and is processed as it flows therebetween. Flow-through MFCs can be implemented in a number of different ways. For example, in FIG. 1 a nutrient is added to the solution 190 through an inlet to the anode chamber 120 and nutrient-depleted solution 190 is emptied from an outlet of the anode chamber 120. The solution 190 in the cathode chamber 130 can be similarly replenished. In the membraneless type of FTMFC the solution 190 flows around the anode 150 and then around the cathode 160 as it flows from the inlet of the MFC to the outlet thereof, aiding the transport of hydrogen ions to the cathode 160.
In flow-through systems, the ability and extent to which nutrients are consumed and processed by the microbes 170 is largely based on the concentration and the metabolic state of the microbes 170, which can vary with position within the MFC 100. In turn, the microbes 170 at any point grow to the concentration which can be supported by the amount of nutrient available there. In those flow-through MFCs that seek to process the nutrient concentration to below some low threshold level, the low nutrient level near the outlet will result in a low microbe population. If the nutrient concentration is below the minimum concentration required to support microbes 170, then microbe population beyond this point will be zero.